The present invention relates to detection of gases, and, more particularly, to the detection of changes in the compositional amounts of gases in air or in a gaseous exhaust product.
Gas detection is important in numerous contexts. For instance, smoke and fire alarms may detect the presence of any of various gases or particles in the air. Carbon dioxide is an example of one of such gases. Carbon dioxide is given off as a bi-product of combustion reactions. Carbon dioxide concentrations in air significantly increase if combustion is occurring in a closed space or nearby. The carbon dioxide given off disperse through the air, and the dispersion may occur more rapidly and in different directions (such a radially outward rather than rising with heat) than smoke or water vapor dispersion. Depending somewhat upon the material and conditions of combustion, smouldering combustion may produced a significant amount of carbon dioxide prior to significant production of smoke and prior to bursting into flames. Accordingly, detecting a rapid increase in the carbon dioxide concentration in the air may signal an alarm condition earlier than possible with conventional smoke detectors.
Gas detection is also significant in other venues. Detection of natural gas or other fuel gases may be important to assure that the natural gas or fuel gas is maintained within the pipe lines without leakage. Carbon monoxide detection is important for health and safety concerns of human breathing. Auto emissions may use gas detectors to verify adequate combustion of the fuel, to minimize hydrocarbons and/or carbon monoxide in resulting exhaust.
Photoacoustic gas detection is known in the art as described for example in U.S. Pat. Nos. 6,006,585, 5,125,749, 4,903,248, 4,886,681, 4,740,086 and 4,236,827, and patents discussed in the background of U.S. Pat. No. 4,094,608. As early as 1880, Alexander Graham Bell reported the acoustic effect induced by thin discs to interrupted beams of light. Photoacoustic gas detection devices use interrupted radiation of a preselected wave length to irradiate or pass through the gas to be measured. Regular modulation of the radiation intensity creates sound waves that can be detected with a very sensitive microphone.
Due to the required sensitivity of the microphone, the molecular sample and the microphones must be heavily insulated to limit uncertainty in the photo acoustic reading due to external acoustic vibrations. While the microphone technology has become increasingly sophisticated, leading to improvements in the sensitivity of the photo acoustic gas detectors, the sensitivity of the gas detector is limited by the sensitivity of the microphones and by the ability to insulate against external acoustic vibrations. In addition, while interference of varying frequencies may be eliminated through various calculations, interference of the same frequency as the acoustic waves formed by the excited molecules in the sample cannot be systematically eliminated by calculation without jeopardizing the accuracy of the reading. Interference at the same frequency as the excited molecular sample would be indistinguishable from the acoustic wave to be measured.
Photoacoustic gas detectors may be fairly expensive, particularly depending upon the required accuracy of the microphone. A highly accurate photoacoustic gas detector would require a great deal of sound insulation to prevent ambient interference. The requirement that the radiation be modulated at an acoustic frequency which can be sensed by the microphone also adds costs. U.S. Pat. No. 4,094,608 shows one alternative, but requires the sample to be absorbed or dissolved or embedded into a layer of electrical insulating material. Thus the sample must be collected prior to spectroanalysis, and the collection procedure must be separately performed. A low cost, accurate, and highly sensitive gas detector, which does not require complicated sample collection, would find a wealth of uses.
The present invention is a photo expansion gas detector, which detects the gas content of its environs by capacitively sensing a changes in position of a diaphragm moved by expansion and/or contraction of a known gas in a sealed container. The photo expansion gas detector, includes a radiation emitter which directs radiation through a sample gas and into an expansion gas chamber. A diaphragm is in contact with the expansion gas and moves when the expansion gas expands or contracts. The capacitive diaphragm deflects relative to a fixed capacitive plate, resulting in changes in capacitance representing expansion and contraction of the expansion gas. The electrical signal generated by the changes in capacitance represents changes in the gas composition of the sample.